Sounds like you didn't get the operator manual with your GT275.
The answers to your questions are in the manual.

It doesn't seem to be available on the JD site for free so you have to order it and pay. A worthwhile investment.

click here

Raise the deck all the way up, then turn the round height adjuster knob all the way to 1 or blank, then lower the deck all the way down. You will have a tension release lever (LH side) that will allow you to roll off the primary belt . Just unhook and draw out and back. Unhook the draft arms in the front- typically unsnap with a noticeable lever on the LH side underneath the front bumper. Pull out the the hook at the rear of the deck on both sides and disengage the pin from the lift arm and let it rest as so it doesn't protrude through the hole. Put the deck control in the highest position and finagle the deck out from the underside of the tractor. Should you need more info, let us know which deck your tractor has. Before leveling, air the tires usually 10 front/ 14 rear which you can vary to your tastes -just have them equal side to side. Each rear lift arm has an adjuster. Low tire pressure and bent blades are the normal culprits. Congrats. You have a nice ride!

Ditto on making sure the tires are properly inflated BEFORE doing any mechanical adjustments to level the deck.
Under inflated tires cause the chassis to "squat" or lean toward the lowest pressure tire even in a straight line run.
Low tire pressure in turns is a double whammy in that the rolling resistance in turns will cause the outside tires to try and "roll under", making their height even lower while the inside tires actually get a bit taller because the weight shift due to the turn puts less weight on the inside tires.

JD sells a blade adjustment gauge for around $15. With the tires properly inflated, the mower on a flat surface and the deck down to the cutting height level, you can measure the blade height to see if the side to side is the same. It can also be used to measure the blade height angle front to back. The front of the blade is usually lower in the front than in the back, depends on the what the setup defines. I think the 275 is around 1/4 inch lower in the front than in the back, the manual should tell.

I have four of the triangle three wheel caster assemblies, the type used to move furniture. Before I start the removal, I place one under each corner of the deck and then as said, set the mower to a 1" cut and lower the deck onto the casters. Remove the pins from the two rear attachment points, the bar at the front and disconnect the arm that activates the blades.
Move the lever to the position that would raise the mower, if attached, raise the front of the tractor and the deck will slide out easily.

I have a floor jack that I use, but other types of jacks could be us.

I also use the floor jack to lift the front wheels off of the floor so that the wheels turn easily, so you can check the air in the front wheels.

thanks guys !

i will tackle it this weekend coming up.

so i still have a question, regarding the grass by the curb, where its on a small slope, can i cut with a rider there or wil is always sclap the lawn because its not on a flat surface?